I have a flexdeck. It kicks a$$. Need i say more?
OK, so you can't use it without a kite very well, if at all, but it does what it is made for perfectly. I have never ridden an ATB before, only blades (inline skates, not the damned kite;) ), and i found that i could stand on it as soon as i stepped on it. After about 15-20 minutes of use, i could go in a straight line and turn a little as well. However, the last 5 minutes of those 20 were the most crucial, i got on it and went like a bullet (must of been going pretty quick i guess), as they were when i mastered the art of the straight line lol. The first 15 i kept falling off because there was no wind on the days when i used it.
Basically, if you have a semi-decent sense of balance, you'll be able to get going in a straight line in minutes with the right wind and kite (with a flexdeck).
Haven't been on it since, but i can't wait til i get the chance
Flexdeck rules:)